CNIDARIA 



89 



ously arisen from cells, bat which now showed that it was 

 composed of nuclei and fat spherules only. A similar struc- 

 ture of the planula is also described for Actinia aurantiaca 

 and Balanophyllia regia ; Jourdan's observations show, how- 

 ever, that from the presence of an inner mass filling up the 

 planula we ai-e not at all justified in inferring the origin of 

 the mass from a solid morula. Balfour refers to observa- 

 tions of Kleinenberg according to which the cleavage of the 

 Zoantharia is frequently unequal ; this would allow one to 

 infer the formation of an epibolic gastrula. Accordingly 

 the formation of the entoderm by delamination from a solid 

 morula in this case still appears doubtful. 



In another series of cases the development of a unilaminar 

 ciliated blastodermic vesicle has been observed, from which 

 the gastrula-stage is produced by invagination; thus in an 

 edible Actinian from Faro (Messina), closely related to 

 Actinia mesembryanthemum, observed by Kowalevskv. 

 Here the blastopore does not close completely, but is directly 

 converted into the inner opening of the oesophagus, while 

 the oesophagus, lined with ectoderm, is developed by the en- 

 folding of the margins of the mouth-opening. In Cerianthus 

 also the formation of a coeloblastula and an invaginate gas- 

 trula following total unequal cleavage was observed by 

 KowALEVSKY. Probably Caryophyllia also belongs here. 



In Actinia equina, according to Jourdan, there is formed a typical in- 

 vaginate gastrula, whose gastral cavity is at first completely empty, and 

 whose entodermal cells contain but little food-yolk. Nevertheless the 

 stomach of the planula larva is filled with coarse yolk granules It still 

 remains uncertain whether these are produced by secretion or by the 

 partial disintegration of the cells of the entoderm. 



According to the observations of H. V. Wilson on Manicina areolata, 

 first a cceloblastula is formed by total cleavage. Then, by the transverse 

 division of the tall cells of the blastosiihere — consequently by delamina- 

 tion— coarsely granular cells are repeatedly constricted off, and finally 

 fill completely the cleavage cavity. While the ectoderm becomes some- 

 what more shai-ply marked off from the inner cell-mass, the oesophageal 

 invagination arises. The larva now becomes covered with cilia and 

 swims about. The permanent entoderm arises, as in the Alcyonaria, 

 from the inner cell-mass, the cells lying next to the ectoderm arranging 

 themselves into an epithelium, while the central mass is finally resorbed. 



At any rate, through these various processes of develop- 



